These flap discs are made of Zirconia. What does that mean?

How a flap disc performs is largely related to the abrasive material used and the grit size. There are three primary cutting grains used: aluminum oxide, zirconia alumina and ceramic alumina. The flaps wear away as the grains deplete, which exposes fresh and sharp grains below. Zirconia discs provide the most versatility for the lowest cost benefit. Although it is slightly more expensive upfront than an aluminium oxide flap disc, zirconia is more heat resistant, tough and durable, and maintains its sharpness for a longer period of time. Zirconia discs will last at least three times as long as the brown aluminium oxide discs.